Apparatus for the preparation of bituminous coatings for road surfaces from virgin aggregates, liquid bitumen and dry bulk materials, in which recycled spent bituminous coatings are incorporated, are known in the art. These usually consist of a large cylindrical mounted on a platform for rotation about its axis and inclined slightly relative to the horizontal plane. The virgin aggregates and the dry bulk materials are introduced into the drum at one of its ends. The coatings recycled in granular form are introduced via a recycling ring surrounding the drum in an intermediate zone between its two ends.
A burner enters the drum at one of its ends and makes it possible to circulate hot gases which ensure the drying and heating of the materials circulating in the drum.
Such a drum ensures at the same time the drying and heating of the cold and wet aggregates entering the drum, reheating of the recycled coatings and mixing of the virgin aggregates and recycled coatings, in contact with liquid bitumen conveyed into the drum by means of an injection pipe.
The inner wall of the drum is fitted with blades of different shapes according to the zones of the drum, in order, as a result of the rotation of the drum, to ensure transport, agitation and/or clearance of the materials circulating in the drum. In comparison with an older process in which the drying and heating of the aggregates took place in a rotary dryer and the mixing with the liquid bitumen was carried out in an independent mixer having a stationary casing, the integration of the drying, heating and mixing functions in one and the same drum afforded a certain degree of simplification of the procedures and equipment used. However, the disadvantage of drying/mixing drums is that a flame, very hot gases and liquid bitumen coexist in the same containment. Because of this, bitumen vapor is carried along by the hot gases circulating in the drum, thereby producing rapid clogging of the back filters used for removing dust from the gases leaving the drum and discharge of harmful vapors into the atmosphere. These disadvantages are especially pronounced in drying/mixing drums with parallel flows, i.e., those where the hot gases circulate inside the drum in the same direction as the solid materials.
Furthermore, any additives to the coatings, for example in the form of powders or fibers, must either be introduced into the drum at one of its two ends, or it is necessary to use an auxiliary device, such as the recycling ring employed for introducing recycled bituminous coatings into the drum, in an intermediate zone between its two ends.
It is not desirable to introduce these additives at the inlet end of the drum, inasmuch as this inlet is usually a long distance from the mixing zone.
Introduction at the outlet end of the drum requires the use of pipes or other devices for the introduction of materials, which are extended inside the drum as far as the entrance of the mixing zone. This complicates the design and the procedures for using the drum.
The recycling ring employed for introducing spent coatings in granular form into the drum is not a suitable means for introducing the dry bulk materials or fibers at the entrance of the mixing zone.
Conversely, in the case of mixers having a stationary casing, it is very easy to place a device for the introduction of dry bulk materials or fibers at any location on such casing, in order to administer the additives at the desired moment, at the start of or during mixing.
The devices employing a separate dryer and mixer with stationary casing therefore have the advantage of reducing the formation and discharges of bitumen vapor and making it easier to administer additives to the coated products during the mixing stage.
However, for a given production capacity, such installations are more complex, more bulky and usually more costly, than the drying/mixing drums.
Moreover, the total motive power necessary for the dryer and mixer of an installation of the old design is markedly higher than the motive power necessary for a drying/mixing drum of the same capacity.
Besides, the installations having a separate dryer and mixer are difficult to use for producing bituminous coatings from virgin aggregates and bitumen, in which a certain proportion of recycled coatings is incorporated.
US-A-2,42l,345 discloses a drying drum in which the hot gases coming from the burner and the aggregates circulate in opposite directions, Furthermore, the end of the drum opposite the inlet end for the aggregates communicates with a mixer having a stationary casing, into which the aggregates pour at the outlet of the drum. The drum is extended by longitudinal supports carrying agitating vanes which move inside the stationary-casing mixer during the rotation of the drum. Liquid bitumen is incorporated in the aggregates in the mixer. In such an apparatus, the burner flame is generated in the mixing zone, thereby giving rise to the formation of bitumen vapors, even if a cylindrical chamber is provided at the center of the casing of the mixer in order to prevent direct contact between the bitumen and the flame.
Moreover, the appliance according to US-A2,421,345 does not make it possible to incorporate spent coatings in the materials circulating in the drum.